Journal and Proceedings of The Royal Society of New South Wales Volume 120 Parts 1 and 2 [Issued September, 1987]
Selection, Adaption and Evolution
R. H. Crozier
Nothing in biology makes any sense except in the light of evolution. (Dobzhansky, 1973)
In a way, evolution proceeds like a tinkerer who, during millions of years, has slowly modified his products, retouching, cutting, lengthening, using all opportunities to transform and create. Jacob (1983)
ON THE SHOULDERS OF A GIANT
As Dobzhansky (1973) so rightly stressed, the realization of evolution forms the very core of modern biology. We are used to understanding how good “design” of organisms comes about through evolution, but sometimes forget that design flaws are also understandable as arising the same way. As Jacob stresses, the processes of evolution work with what has gone before rather than producing new organisms afresh when faced with new niches or other evolutionary opportunities. Dawkins (1982) notes two examples of poor design whose origins are readily understandable in evolutionary terms: the contorted faces of flatfish, and the long detour taken by the recurrent laryngeal nerve in a giraffe’s neck. It is no wonder, even leaving aside the evidence from natural and experimental populations, that scientists abandoned special creation of any kind as a general explanation for the living world.
As Stent (1972) notes, progress in science, as in art, has a certain inevitable aspect to it: an advance not made by person A today will be made by person B tomorrow. But occasional individuals make their advances with such thoroughness that we can be certain that, without them, we would have to have waited much longer to reach where we are today. Such a person was Charles Darwin, who not only generated an explanatory theory for evolution that is quite modern, but amassed such a treasure trove of data that the survival of the new paradigm could not be long in doubt. In Simpson’s (1964) phrase, modern biology is truly the “world into which Darwin led us”.
Darwin’s vision and breadth of knowledge were awesome, as anyone reading through his works can testify. Not only did he establish the main outlines of evolution, but he clearly foreshadowed such modern topics as variation in evolutionary rates (as discussed by Penny, 1983), and even the whole field of sociobiology (not only and obviously in “The descent of man” but also in his discussions of social insects in “The origin of species”).
Darwin’s vision was far-reaching, but today, standing on his shoulders (to paraphrase Newton), we see much farther than he could possibly do (a point sometimes missed by philosophers of science). To acknowledge that our vision is different, even for concepts such as natural selection, clearly does nothing to reduce our appreciation of Darwin: stasis in evolutionary understanding would be a failure, and Darwin did not fail.
GENETICS, CHANCE, AND NECESSITY
The greatest single cause of advance in understanding evolutionary mechanisms since Darwin has been the rise of genetics. Evolution is clearly understood now as genetic change in a lineage, and evolutionists generally engage in studying the causes and consequences of such change. Both cause and consequence can, of course, be complex, as I will try to make clear below.
Is there a role for chance in this view of life’s history? Yes, and not just one role. One role is that of constant origin through mutation of new alleles differing so trivially in effect from others that their fixation is due to chance. Such a process is envisaged by the Neutralist school of molecular evolutionists as the main one giving rise to molecular change (Kimura, 1983), whereas other evolutionists suggest that the neutralist domain is relatively minor.
The other role for chance concerns the ultimate effects of early choices: whether a population takes one fork in the evolutionary road instead of another may be a matter of chance, yet the two roads may lead to quite different results. Such early choices have a considerable effect, not only on the adaptations that are later available to the population, but also on the way in which different organisms adapt. Monocotyledons and dicotyledons have both evolved tree growth forms on occasion, but in different ways: palms are of necessity constructed very differently from true trees because they are monocotyledons, and monocotyledons lack the ability for secondary strengthening expressed by dicotyledons (Maynard Smith et al., 1985).
Monocotyledons have to some extent overcome the basic constraints of their architecture by evolving tree forms in their own way, but Gould (e.g., 1980) has frequently stressed that such constraints will often be insuperable, limiting the possible array of forms. The concept of constraint is a useful one, in that it must sometimes be correct (Bull and Charnov, 1985) and in that it can lead to tests and be falsified on occasion. For example, LaBarbera (1983) showed that, contrary to general belief, the scarcity of wheels among higher organisms is not because they can’t evolve them, but because they are less efficient than the observed forms of transport (moreover, where they are more efficient, they have in fact evolved). Apart from neutralist explanations for morphology and life-pattern, there are therefore three basic kinds of explanation of why certain features are seen and others are not (Crozier and Page, 1985): structuralist (absent features are either impossible or the way to them is blocked by impossible intermediates), historicist (there has not been sufficient time for the missing features to have been evolved, or only some of the adaptive peaks are occupied so far), and, of course, adaptationist (missing features confer less fitness than the ones that are present). Structuralist and adaptationist examples overlap, because an impossible phenotype is certainly also lethal, and nearby phenotypes, even if not impossible, are almost certainly of low fitness. For example, a clam shell which cannot open would be a lethal combination, and ones that can only open a tiny fraction would be of low fitness.
Of necessity, I will not spend further time here discussing chance, but rather concentrate on the process of adaptation, the most interesting aspect of evolution to most people.
База данных защищена авторским правом ©shkola.of.by 2016
звярнуцца да адміністрацыі